Method for operating a sensor for determining the concentration of oxidizing gases in gas mixtures

ABSTRACT

Method for operating a sensor for determining the concentration of oxidizing gases in gas mixtures, especially of the nitrogen oxide concentration in exhaust gases of internal combustion engines, wherein the sensor includes: at least one chamber ( 1; 2 ) mounted in a solid state electrolyte ( 20 ), the chamber being connected to the gas mixture via a first diffusion barrier ( 4 ); a second chamber ( 3 ) arranged in the solid state electrolyte ( 20 ) and the chamber having a pregivable constant oxygen partial pressure; on the solid state electrolyte, an oxygen pump electrode ( 9 ) subjected to the exhaust gas; a further oxygen pump electrode ( 7; 8 ) as well as an NO pump electrode ( 10 ) in the at least one chamber ( 1; 2 ); and an oxygen reference electrode ( 6 ) arranged in the second chamber ( 3 ); and at least a voltage is made available at the electrodes and at least a pump current is evaluated as a measurement signal, is characterized in that the voltages (U_IPE; U_O2; U_NO), which are applied to the electrodes, are changed in dependence upon the currents, which flow in the electrode feed lines and/or between the electrodes ( 6; 7; 8; 9; 10 ), during operation of the sensor in such a manner that the voltages correspond to pregivable desired values, these voltages being applied to the electrodes ( 6; 7; 8; 9; 10 ) in the interior of the sensor.

[0001] The invention relates to a method for operating a sensor fordetermining the concentration of oxidizing gases, especially fordetermining the nitrogen oxide concentration in exhaust gases ofinternal combustion engines in accordance with the preamble of claim 1.

STATE OF THE ART

[0002] Such a sensor is presented, for example, in EP 0 791 826 A1.

[0003] Electrical fields and electrical currents arise between theindividual electrodes and between the electrodes and the heater whichcause the measuring result to be incorrect. This occurs because allelectrodes of such a sensor are conductively is connected to the solidstate electrolyte and the insulation layer of the heater has a finiteresistance and, accordingly, all electrodes are connected to each othervia electrically conductive structures and are connected at highresistance to the heater.

ADVANTAGE OF THE INVENTION

[0004] The method of the invention having the features set forth inclaim 1, in contrast, offers the advantage that the measuring errors canbe eliminated by active compensation or at least can be minimized. Themeasurement errors arise because of the mutual coupling of theelectrodes via electrical fields and currents in the solid stateelectrolyte as well as by the voltage drops acrcss the feed lineresistances. It is possible to precisely adjust the voltages on theelectrodes without errors being made incorrect by voltage drops on theelectrode feed lines or because of a mutual coupling of electrodes. Thisis made possible by changing the voltages, which are applied to theelectrodes in accordance with function, in dependence upon the currentswhich flow in the electrode feed lines and/or between the electrodes. Itis especially advantageous that the adjustment is independent of thecurrent intensity with which the individual electrodes are charged.

[0005] An advantageous embodiment provides that one adds voltages to thevoltages applied to the electrodes. The voltages added correspond to afeedback of voltage components weighted with factors and these voltagecomponents are proportional to the currents. Furthermore, the slidingmean values of the voltages and/or their derivation of higher orderand/or their sliding mean values or linear combinations thereof can befed back. These voltages are proportional to the currents and the meanvalues are is formed by means of known electric circuit elements. Inthis way, it is also possible to eliminate capacitive couplings.

[0006] The adjustment of the voltage on the electrodes takes place inthis case advantageously by changing these factors. These factors areincreased until the system starts to oscillate because of the feedback.The oscillation arises when the fed back factor is ≧1 in magnitude and,at the same time, the phase is greater or equal to 180°. Then, thefactors are reduced slightly but only so far that just no oscillationoccurs anymore. In this way, almost all voltage drops, which arise atthe electrode feed lines, as well as the voltage drops which arisebecause of a fictive resistance network within the solid stateelectrolytes, can be compensated.

DRAWING

[0007] Further advantages and features of the invention are the subjectmatter of the following description as well as the schematicrepresentation of an embodiment of the invention.

[0008] The drawings show:

[0009]FIG. 1 is a schematic section view through a sensor, which isknown from the state of the art, for determining oxides in gas mixtures;

[0010]FIG. 2 shows schematically a circuit arrangement, which is knownfrom the state of the art, for a sensor shown in FIG. 1;

[0011]FIG. 3 is an embodiment of a circuit arrangement for a sensorshown in FIG. 1 which is suitable for carrying out the method of theinvention; and,

[0012]FIG. 4 schematically shows the coupling of the voltages/currentsin matrix form which lie across the electrodes of a sensor shown in FIG.1.

DESCRIPTION OF THE EMBODIMENTS

[0013] An NOx double chamber sensor is shown in FIG. 1 and includes fiveelectrodes, namely: an oxygen pump electrode 9 subjected to the exhaustgas; an oxygen pump electrode 7 mounted in a first chamber andessentially lying opposite to the oxygen pump electrode 9 subjected tothe exhaust gas; an oxygen pump electrode 8 arranged in a second chamber2; and NO pump electrode 10 mounted likewise in the second chamber 2;and, an air reference electrode 6 mounted in a third chamber 3.

[0014] The first chamber 1 is connected via a diffusion barrier 4 to theexhaust gas, the second chamber 2 is connected to the first chamber viaa further diffusion barrier 5.

[0015] The third chamber 3 is connected to the atmosphere via a channel.

[0016] The oxygen pump electrodes 7 and 8 pump oxygen away from thefirst chamber 1 or from the second chamber 2. The external pumpelectrode 9 functions as a counter electrode.

[0017] Nitrogen oxides are pumped away by the NO pump electrode 10. Allelectrodes are arranged on an ion-conducting solid state electrolyte 20which, for example, can be made of zirconium oxide and are electricallyconductively connected therewith.

[0018] An insulated heater 11 is provided in order to heat up the sensorto the necessary operating temperature.

[0019] An evaluation circuit functions to operate the sensor and thiscircuit makes various electrical voltages available and obtains themeasurement signal from a current measurement. A block circuit diagramof such a circuit, which is known from the state of the art, is shownschematically in FIG. 2. The three voltages for the oxygen pumpelectrodes (7, 8) as well as for the NO pump electrode 10 are generatedby voltage references 31, 32, 33 and drivers 41, 42, 43 and are shiftedby the potential of the air reference. The oxygen pump electrodes 7, 8lie in the first chamber 1 and in the second chamber 2. For the above,the voltage, which is outputted by the driver 40, is added to orsubtracted from the voltage outputted by drivers 41, 42, 43 in addingelements 61, 62, 63 in a manner known per se. The potential of the outerpump electrode 9 is adjusted via a two-point controller 50 until thevoltage difference between the oxygen pump electrode 7 and the airreference electrode 6 corresponds to a pregivable desired value. Theother electrode potentials are adjusted directly. The NO pump currentcan be measured via a current-voltage converter 80 known per se and beoutputted as a measurement signal.

[0020] All electrodes are conductively connected to the solid stateelectrolyte 20 and the insulation layer of the heater 11 has a finiteresistance. For this reason, all electrodes are connected to each othervia a conductance network and are connected at high ohmage to the heater11. The numerically largest conductances are shown in FIG. 1schematically by the resistances R_(E). Likewise, feed conductances ofthe conductive paths to the electrodes are present which are likewiseshown schematically in FIG. 1 by resistances R_(L).

[0021] The basic idea of the invention is to make possible theadjustment of the required voltages directly at the electrodes withoutthe voltage drop across the feed line resistors R_(L) or the mutualcoupling of the electrodes via the resistances Rs making these electrodevoltages incorrect.

[0022] This is solved by a method for operating a sensor which isexplained in combination with a circuit shown in FIG. 3. In the circuitshown in FIG. 3, those elements which are identical to those in thecircuit shown in FIG. 2 have the same reference numerals so that, withreference to their description, reference is made to the presentationmade to the circuit shown in FIG. 2. The circuit shown in FIG. 3 differsfrom the circuit shown in FIG. 2 in that circuit arrangements areprovided by which the voltages U_IPE, U_NO, U_O2, which are applied tothe electrodes 7, 8, 10, 9, are changeable in dependence upon thecurrents flowing in the measurement lines and/or between the electrodes.These circuit arrangements include current/voltage converters 100, 110,120 and circuit elements (compensation branches) 201, 202, 203, 204,205, 206 which are weighted with compensation factors K1, K2, K3, K4,K5, K6 in such a manner that a component, which is proportional to thecurrents, is so fed back to the electrodes that the components, whichare coupled in via the solid state electrolyte 20, and the feed lossesare compensated. with a feedback of this kind, the potentials of theelectrodes, which can be measured on the feed lines, are dependent uponthe currents in the solid state electrolyte 20 and in the feed lines.The currents in the solid state electrolytes 20 are not accessible to ameasurement but result at every location from the linear combination ofcurrents in the feed lines. The total system is viewed as being linearlyelectrical. Because of the linear combination of the currents at eachlocation, one obtains voltages also at the locations of the electrodeswhich are linearly dependent upon the feed line currents. The feedbacktakes place in such a manner that first the factor K1 is increasedstepwise until there is an oscillation because of the feedback. Then,the factor K1 is again reduced slightly until just no oscillationoccurs. Correspondingly, and if still necessary, one can proceed withthe additional factors K2 to K5. In this way, it is ensured thatpractically all disturbing influences because of the electrode feedlines and because of the resistances between the electrodes areeliminated. These resistances are present in the solid state electrolyte20 and are disturbing. Additionally, the sliding mean values of thevoltages formed by means of electrical circuit elements and/or theirderivatives of higher order and/or their sliding mean values or linearcombinations thereof can be fed back. The voltages are proportional tothe current. In this way, not only ohmic but also capacitive couplingsare eliminated.

[0023]FIG. 4 shows schematically the coupling matrix. The lines areformed by the currents of the electrodes I_pump electrode 7, I_O2 pumpelectrode 8 and I_NO pump electrode 10. The current of the inner oxygenpump electrode I_pump electrode 7 is relatively large compared to theother two and has therefore significant influence on the electrodevoltages U_IPE or on the pump electrode 7, U_O2 on the pump electrode 8and U_NO on the pump electrode 10. The spatial closeness of the oxygenpump electrode 8 and the NO pump electrode 10 to each other in thesecond chamber 2 leads to a pronounced coupling. The components of themain diagonal of the coupling matrix result from the feed lineresistances. Since the matrix is symmetrical, it is sufficient toconsider the compensation factors K2, K3, K5 and K1, K4, K6 arranged onone side of the main diagonal.

1. Method for operating a sensor for determining the concentration ofoxidizing gases in gas mixtures, especially of the nitrogen oxideconcentration in exhaust gases of internal combustion engines, whereinthe sensor includes: at least one chamber (1; 2) mounted in a solidstate electrolyte (20), the chamber being connected to the gas mixturevia a first diffusion barrier (4); a second chamber (3) arranged in thesolid state electrolyte (20) and the chamber having a pregivableconstant oxygen partial pressure; on the solid state electrolyte, anoxygen pump electrode (9) subjected to the exhaust gas; a further oxygenpump electrode (7; 8) as well as an NO pump electrode (10) in the atleast one chamber (1; 2); and an oxygen reference electrode (6) arrangedin the second chamber (3); and at least a voltage is made available atthe electrodes and at least a pump current is evaluated as a measurementsignal, characterized in that: the voltages (U_IPE; U_O2; U_NO), whichare applied to the electrodes, are changed in dependence upon thecurrents, which flow in the electrode feed lines and/or between theelectrodes (6; 7; 8; 9; 10), during operation of the sensor in such amanner that the voltages correspond to pregivable desired values, thesevoltages being applied to the electrodes (6; 7; 8; 9; 10) in theinterior of the sensor.
 2. Method of claim 1, characterized in that oneadds voltages to the voltages applied to the electrodes, these addedvoltages corresponding to a feedback of voltage components weighted withfactors (K1, K2, K3, K4, K5, K6) which voltage components areproportional to the currents, which flow in the electrode feed linesand/or between the electrodes (6; 7; 8; 9; 10) during operation of thesensor and/or are proportional to the sliding mean values of thevoltages, which are proportional to the currents and which are formed bymeans of electric circuit elements and/or the derivatives of higherorder and/or their sliding mean values or linear combinations thereof.3. Method of claim 1 or 2, characterized in that one increases at leastone of the factors (K1, K2, K3, K4, K5, K6) so long until an oscillationoccurs because of the feedback and that one slightly reduces this factor(K1, K2, K3, K4, K5, K6) by an amount determined experimentally so thatjust no oscillation occurs anymore.